1. Field of the Invention
This invention relates to a method for beneficiating a non-homogeneous agglomeration of coal refuse that include carbonaceous and clay particles having a natural affinity for retention of free water. More particularly, the present invention provides a method for beneficiating such particles by rapidly heating the coal refuse to vaporize and release free water and to subdivide the agglomeration forming the coal refuse so that classification procedures can be employed to derive a particle fraction having a Btu value suitable to form a low or clay free fuel supply.
2. Field of the Invention
While not so limited, the present invention can be used to provide a fuel supply for a heat generating combustor utilizing fluidized bed combustion technology which permits the use of fuel derived from many forms of waste materials having a residual heat value substantially lower than could be economically utilized in the past. According to the present state of the art, fluidized bed combustors can economically function with a fuel having a uniform heat value of as low as 3,000 Btu per pound. The solid waste stream generated by the operation of coal processing plant forms an abundant source of waste coal refuse fuel. Large surface deposits, often abandoned, occur due to the accumulation of coal refuse. The coal refuse is a reject remnant of coal processing plants with a high hourly through-put processing rate coupled with a multi-decade life span. Such accumulations can readily yield a fuel having a total usable heat value of a magnitude sufficient to meet the requirement for fueling a large fluidized-bed combustor. Such a combustor may be capable of supplying the energy necessary to generate 30 to 80 megawatts of electrical power throughout a useable life span of the generating facility. The deposits can be found where coal has been mined and prepared and therefore the deposits are generally widespread. A principle contaminant of carbonaceous deposits, particularly remnants from coal cleaning operations, occurs as rock formations composed principally of clay material of hydrus-alumina-silicate composition with a relatively uniform particle size in the range of 0.7 to 2 micrometers. The abundance of such clay material dispersed in the carbonaceous component of the refuse is not only widely varied but also a major component. Contributing rocks and/or minerals in a pure state with a high clay content are characterized by a dry ash content of more than 80 percent accompanied by a heat value of the refuse material of less than 1,500 Btu's per dry pound of refuse. The particles of rocks and minerals that are non-contributors to the usable heat include clay; shale, which may comprise clay shale or sandy shale; sand stone; lime stone and lime stone/calcite. Such rock/minerals are subject, through the slaking process, to disintegration and quickly pass from a en masse rock form into an agglomerated matri of individual grains capable of plastic flow. The present invention provides a process for effectively beneficiating refuse deposits having heretofore uncommercially recoverable amounts of carbonaceous components by utilizing the phenomena of the uniform particle size of the clay component.
In coal refuse deposits discussed hereinbefore, the average heat value of the deposits is a function of the combustible content of various forms of rock or mineral matter that constitute the total of the deposit. The average usable heat value is the aggregate of the calorific contributions due to the presence of particles of pure coal, pure coal/bone, carbonaceous shales and sulfur bearing minerals, such a pyrite and marcasite. For use as a fuel, the contribution must have a heating value in excess of 1,500 Btu's per dry pound thus the dry ash content less than 80 percent. On the other hand particles of rock or minerals that are non-contributors to useable heat value include clay, shale, sandstone and limestone or calcite which in their pure state are characterized by an ash content of more than 80 percent and an accompanying heat value of less than 1,500 Btu's per dry pound. At any given location in a coal refuse deposit the in situ heat value of a sample is a function of the quantitative distribution or ratio of heat contributing minerals to non-heat contributing minerals. The relativity of particle distribution is considerably, and therefore there is a wide variation to in situ heat values of refuse samples thereby variations to useable heat valve occur in any given coal refuse deposit.
The slaking/weathering process brings about an immediate and drastic alteration to the physical properties of freshly mined clays and clay shales upon placement of a reject product in a coal refuse site. Such freshly mined clay and clay shales are part of contributing and non-contributing rocks and minerals. In refuse coal deposits, platelets of clay occur within a size range of from about 0.7 to about 2 micrometers. Solid freshly mined clay is transformed by the slaking process from an identifiable rock having a weak compressive strength into a plastic mass with little bonding between the clay platelets and therefore capable of plastic flow. As the slaking process proceeds, the high clay content of the refuse pile is compressed due to the weight of the overlying material along with any compressive action due to wheeled vehicles used in the coal refuse placement process. Plastic flow of the micro particles of the clay is induced which brings about a partial or a total encasement or encapsulation of individual macro pieces of other minerals with the clay. The action effectively transforms the coal refuse from an unconsolidated or loose mass of individual particles at the time of placement to a semi-solidified matrix of sticky, agglomerated micro and macro particles having difficult material handling characteristics. As the depth of the high clay content coal refuse deposit increases, the surface moisture content decreases and the density increases. The surface moisture content will generally fall within a range from a maximum of about 15 percent to a minimum of about 5 percent by weight. The total density will vary from as low a minimum of about 90 up to in excess of 110 pounds per cubic foot. The dry density will vary from a minimum of about 80 up to and perhaps in excess of 100 pounds per cubic foot.